Balancing effluent quality, economic cost and greenhouse gas emissions during the evaluation of (plant-wide) control/operational strategies in WWTPs

The carbon footprint of sludge treatment and disposal is typically studied concerning indirect emission sources and fugitive emissions from anaerobic digestion. Whereas greenhouse gas (GHG) emissions from other sludge treatment units in wastewater treatment plants are often overlooked. In this study, we employed an integrated approach using high-resolution trace gas spectroscopy and a steady-state mass balance method to quantify the GHG emission intensities of the frequently neglected mechanical sludge treatment processes. The results demonstrated that these previously underreported GHG emissions are substantial, with their carbon equivalence being comparable to that of known sources in sludge treatment. Notably, centrifugal dewatering was identified as the predominant source of GHG emissions. Moreover, overall GHG emissions are higher in winter (1.80 × 10-2 kgCO2eq/kgTSS) than in summer (1.33 × 10-2 kgCO2eq/kgTSS). The GHG emissions during sludge treatment exhibited distinct pathways: methane was released both in situ (driven by biological activity) and ex situ (induced by mechanical perturbation), while nitrous oxide production primarily originated from in situ biological processes. This study enhances the understanding of GHG emissions during sludge treatment and establishes a tiered accounting framework for sludge carbon emission, thereby facilitating low-carbon sludge treatment.

Dairy production systems represent a significant source of air pollutants such as greenhouse gases (GHG), that increase global warming, and ammonia (NH3), that leads to eutrophication and acidification of natural ecosystems. Greenhouse gases and ammonia are emitted both by conventional and organic dairy systems. Several studies have already been conducted to design practices that reduce greenhouse gas and ammonia emissions from dairy systems. However, those studies did not consider options specifically applied to organic farming, as well as the multiple trade-offs occurring between these air pollutants. This article reviews agricultural practices that mitigate greenhouse gas and ammonia emissions. Those practices can be applied to the most common organic dairy systems in northern Europe such as organic mixed crop-dairy systems. The following major points of mitigation options for animal production, crop production and grasslands are discussed. Animal production: the most promising options for reducing greenhouse gas emissions at the livestock management level involve either the improvement of animal production through dietary changes and genetic improvement or the reduction of the replacement rate. The control of the protein intake of animals is an effective means to reduce gaseous emissions of nitrogen, but it is difficult to implement in organic dairy farming systems. Considering the manure handling chain, mitigation options involve housing, storage and application. For housing, an increase in the amounts of straw used for bedding reduces NH3 emissions, while the limitation of CH4 emissions from deep litter is achieved by avoiding anaerobic conditions. During the storage of solid manure, composting could be an efficient mitigation option, depending on its management. Addition of straw to solid manure was shown to reduce CH4 and N2O emissions from the manure heaps. During the storage of liquid manure, emptying the slurry store before late spring is an efficient mitigation option to limit both CH4 and NH3 emissions. Addition of a wooden cover also reduces these emissions more efficiently than a natural surface crust alone, but may increase N2O emissions. Anaerobic digestion is the most promising way to reduce the overall greenhouse gas emissions from storage and land spreading, without increasing NH3 emissions. At the application stage, NH3 emissions may be reduced by spreading manure during the coolest part of the day, incorporating it quickly and in narrow bands. Crop production: the mitigation options for crop production focus on limiting CO2 and N2O emissions. The introduction of perennial crops or temporary leys of longer duration are promising options to limit CO2 emissions by storing carbon in plants or soils. Reduced tillage or no tillage as well as the incorporation of crop residues also favour carbon sequestration in soils, but these practices may enhance N2O emissions. Besides, the improvement of crop N-use efficiency through effective management of manure and slurry, by growing catch crops or by delaying the ploughing of leys, is of prime importance to reduce N2O emissions. Grassland: concerning grassland and grazing management, permanent conversion from arable to grassland provides high soil carbon sequestration while increasing or decreasing the livestock density seems not to be an appropriate mitigation option. From the study of the multiple interrelations between gases and between farm compartments, the following mitigation options are advised for organic mixed crop-dairy systems: (1) actions for increasing energy efficiency or fuel savings because they are beneficial in any case, (2) techniques improving efficiency of N management at field and farm levels because they affect not only N2O and NH3 emissions, but also nitrate leaching, and (3) biogas production through anaerobic digestion of manure because it is a promising efficient method to mitigate greenhouse gas emissions, even if the profitability of this expensive investment needs to be carefully studied. Finally, the way the farmer implements the mitigation options, i.e. his practices, will be a determining factor in the reduction of greenhouse gas and NH3 emissions.

Understanding the greenhouse gas emissions from China’s wastewater treatment plants: Based on life cycle assessment coupled with statistical data

This study analyzed greenhouse gas (GHG) emissions from a wastewater treatment plant (WWTP) that uses a combination of physical, chemical, and biological processes and estimated the emissions generated from treatment of oil-rich wastewater from a locomotive repair factory in China. The WWTP produces 526.8 t CO2-equivalent/a corresponding to 4.3 t CO2-equivalent/t oil removed. The combustion of fossil fuels for onsite energy generation is the major source of GHG, accounting for 79.7% of overall emissions. Use of chemicals for metal cleaning, flocculation, and pH control accounts for 13.4% emissions; anaerobic digestion accounts for 3.8% emissions; and the transport of solid waste and subsequent generation of landfill biogas account for 3.1% emissions. Theoretical analysis of various process design alternatives demonstrated that the recovery of biogas produced during anaerobic sludge digestion and its use as fuel reduces the emissions of GHG by 93.9 t CO2-equivalent/a, which is 15.1% of the overall emissions of the treatment plant. The use of aerobic digestion instead of anaerobic digestion in this plant did not significantly effect GHG emissions. Using anaerobic digestion for sludge treatment and releasing the generated CH4 into the atmosphere without further flaring or recovery increased GHG emissions the greatest. The reuse of waste oil and proper management of solid waste are recommended as effective ways of reducing GHG emissions.

Biosolid Management Options in Cassava Starch Industries of Thailand: Present Practice and Future Possibilities

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Sanitation service chains (SSCs) often consist of a complex mix of different components, frequently involving the coexistence of non-sewered sanitation (e.g., septic tanks) and sewered sanitation. Poorly-maintained components within these chains can lead to substantial, yet potentially avoidable greenhouse gas (GHG) emissions. In this study, we developed a model for estimating the impact of GHG mitigation measures along SSCs that feature overlapping and poorly maintained non-sewered and sewered sanitation, taking the interdependencies of the GHG emissions of these components into account. To this end, we employed mass balance, empirical emission equations, and a carbon footprint estimation model to estimate GHG emissions by component at baseline and under four mitigation scenarios using an example SSC in Hanoi. The results showed that the SSC is predominantly methane-emitting, with poorly-maintained septic tanks and sewers being the primary contributors to the GHG emissions. Annual septic tank emptying was also identified as an effective strategy for reducing GHG emissions and it accounted for a 31-38 % decline in total emissions relative to baseline emission level. Scenario comparison further showed that removing septic tanks and upgrading sewers, even though associated with a slight increase in N2O emissions from the wastewater treatment plant, offer the greatest long-term mitigation potential, yielding 15-24 % lower emissions than annual emptying septic tanks with sewer upgrades. Additionally, if septic tanks are not removed, they will remain the primary source of GHG emissions even after upgraded sewer and centralized treatment is established. However, in cases where septic tank removal poses social challenges, frequent emptying remained a robust and immediately applicable mitigation option. Overall, this study provides a framework for identifying and quantifying major GHG emission reduction strategies for complex SSCs. Additionally, the results obtained indicated that managing septic tanks and sewers are important climate action strategies for ensuring sustainable city-wide inclusive sanitation.

Dairy production systems represent a significant source of air pollutants such as greenhouse gases (GHG), that increase global warming, and ammonia (NH3), that leads to eutrophication and acidification of natural ecosystems. Greenhouse gases and ammonia are emitted both by conventional and organic dairy systems. Several studies have already been conducted to design practices that reduce greenhouse gas and ammonia emissions from dairy systems. However, those studies did not consider options specifically applied to organic farming, as well as the multiple trade-offs occurring between these air pollutants. This article reviews agricultural practices that mitigate greenhouse gas and ammonia emissions. Those practices can be applied to the most common organic dairy systems in northern Europe such as organic mixed crop-dairy systems. The following major points of mitigation options for animal production, crop production and grasslands are discussed. Animal production: the most promising options for reducing greenhouse gas emissions at the livestock management level involve either the improvement of animal production through dietary changes and genetic improvement or the reduction of the replacement rate. The control of the protein intake of animals is an effective means to reduce gaseous emissions of nitrogen, but it is difficult to implement in organic dairy farming systems. Considering the manure handling chain, mitigation options involve housing, storage and application. For housing, an increase in the amounts of straw used for bedding reduces NH3 emissions, while the limitation of CH4 emissions from deep litter is achieved by avoiding anaerobic conditions. During the storage of solid manure, composting could be an efficient mitigation option, depending on its management. Addition of straw to solid manure was shown to reduce CH4 and N2O emissions from the manure heaps. During the storage of liquid manure, emptying the slurry store before late spring is an efficient mitigation option to limit both CH4 and NH3 emissions. Addition of a wooden cover also reduces these emissions more efficiently than a natural surface crust alone, but may increase N2O emissions. Anaerobic digestion is the most promising way to reduce the overall greenhouse gas emissions from storage and land spreading, without increasing NH3 emissions. At the application stage, NH3 emissions may be reduced by spreading manure during the coolest part of the day, incorporating it quickly and in narrow bands. Crop production: the mitigation options for crop production focus on limiting CO2 and N2O emissions. The introduction of perennial crops or temporary leys of longer duration are promising options to limit CO2 emissions by storing carbon in plants or soils. Reduced tillage or no tillage as well as the incorporation of crop residues also favour carbon sequestration in soils, but these practices may enhance N2O emissions. Besides, the improvement of crop N-use efficiency through effective management of manure and slurry, by growing catch crops or by delaying the ploughing of leys, is of prime importance to reduce N2O emissions. Grassland: concerning grassland and grazing management, permanent conversion from arable to grassland provides high soil carbon sequestration while increasing or decreasing the livestock density seems not to be an appropriate mitigation option. From the study of the multiple interrelations between gases and between farm compartments, the following mitigation options are advised for organic mixed crop-dairy systems: (1) actions for increasing energy efficiency or fuel savings because they are beneficial in any case, (2) techniques improving efficiency of N management at field and farm levels because they affect not only N2O and NH3 emissions, but also nitrate leaching, and (3) biogas production through anaerobic digestion of manure because it is a promising efficient method to mitigate greenhouse gas emissions, even if the profitability of this expensive investment needs to be carefully studied. Finally, the way the farmer implements the mitigation options, i.e. his practices, will be a determining factor in the reduction of greenhouse gas and NH3 emissions.KeywordsAgricultureGreenhouse gasAmmoniaAbatementMixed crop-dairy systemsOrganicLivestockManureGrasslandCarbon storageSoil carbon sequestration

Global economic development has highlighted the issue of climate change, which is one of the most important environmental issues plaguing human beings. It is widely agreed that excessive greenhouse gas (GHG) emissions are important factors contributing to global warming. Many countries have formulated corresponding GHG emission reduction plans to deal with climate change issues. An important GHG emission source is released from sewage-sludge treatment systems. However, there has not been a comprehensive quantitative GHG emissions evaluation system in the case of sewage-sludge treatment systems, due to multiple emission sources, complex processes, and different standards. In previous studies, the Guidelines for National Greenhouse Gas Inventories (Intergovernmental Panel on Climate Change, IPCC, 2006) and Chinese Greenhouse Gas Inventory (National Center for Climate Change Strategy and International Cooperation, NCSC, 2005) were widely applied to estimate GHG emissions from sewage-sludge treatment. However, IPCC does not consider CO2 emissions from sewage treatment, and NCSC does not consider CO2 emissions from the sewage treatment and N2O emissions from sludge treatment. Therefore, the following have been conducted in this study: (1) A GHG estimation model basing on Life Cycle Thinking (LCT) was constructed, and the research objects were CH4, N2O, and CO2 that were produced by the sewage-sludge treatment system. The estimation model of CO2 and N2O, which were ignored in the IPCC report, were analyzed and discussed. The models of the GHG emission estimation were summarized and improved in the urban sewage-sludge treatment system under the different sewage-sludge treatment process scenarios. (2) The GHG emission load of major urban sewage-sludge treatment processes was analyzed, and the level and key links of environmental impacts generated by different processes were identified. This helps to understand and compare the environmental impacts of different treatment processes and provides suggestions for the sustainable development of wastewater treatment processes. (3) The GHG emission characteristics of nine scenarios of different sewage-sludge treatment processes were analyzed, and the environmental impacts caused by energy consumption and chemicals consumption were studied. Consequently, the sewage-sludge treatment process under low carbonization and low environment impact were proposed.

Evaluation of organic waste diversion alternatives for greenhouse gas reduction

Improvement of air flowrate distribution in the nitrification reactor of the waste water treatment plant by effluent quality, energy and greenhouse gas emissions optimization via artificial neural networks models

The purpose of this study is to compare the effect of a reduction in greenhouse gas (GHG) emissions between the combined heat and power (CHP) plant and boiler, which became the main energy-generating facilities of “anaerobic digestion” (AD) biogas produced in Korea, and analyze the GHG emissions in a life cycle. Full-scale data from two Korean “wastewater treatment plants” (WWTPs), which operated boilers and CHP plants fueled by biogas, were used in order to estimate the reduction potential of GHG emissions based on a “life cycle assessment” (LCA) approach. The GHG emissions of biogas energy facilities were divided into pre-manufacturing stages, production stages, pretreatment stages, and combustion stages, and the GHG emissions by stages were calculated by dividing them into Scope1, Scope2, and Scope3. Based on the calculated reduction intensity, a comparison of GHG reduction effects was made by assuming a scenario in which the amount of biogas produced at domestic sewage treatment plants used for boiler heating is replaced by a CHP plant. Four different scenarios for utilizing biogas are considered based on the GHG emission potential of each utilization plant. The biggest reduction was in the scenario of using all of the biogas in CHP plants and heating the anaerobic digester through district heating. GHG emissions in a life cycle were slightly higher in boilers than in CHP plants because GHG emissions generated by pre-treatment facilities were smaller than other emissions, and lower Scope2 emissions in CHP plants were due to their own use of electricity produced. It was confirmed that the CHP plant using biogas is superior to the boiler in terms of GHG reduction in a life cycle.

Thermophilic biological fluidized bed reactor in sludge line reduces greenhouse gas emissions in wastewater treatment system

Life cycle assessment of sludge anaerobic digestion combined with land application treatment route: Greenhouse gas emission and reduction potential

Background: The objective of this study was to establish a facility-level modeling tool to assess the performance of internal operational strategies and external emissions. Methods: The biological process model in Benchmark Simulation Model No. 2 (BSM2) was upgraded to include two-stage and four-stage nitrification, as well as denitrification processes, to effectively simulate nitrous oxide (N2 O) production. Emissions of carbon dioxide (CO2 ), methane (CH4 ), and N2 O were also incorporated, taking into account digestion, cogeneration, and sludge storage processes. The refined model was utilized in a case study at the Isfahan Wastewater Treatment Plant (WWTP) in Iran to assess various operational strategies and explore potential challenges related to emission management. Results: The simulation results, based on the model’s structural assumptions, indicate a negative impact on greenhouse gas (GHG) emissions associated with regional energy upgrades in the ventilation system and activated sludge sector. For example, variations in the biogenic and non-biogenic CO2 proportions were observed when the total suspended solids (TSS) removal efficiency was altered, decreasing or increasing to 30/70 and 20/80, respectively. While off-site CO2 emissions can be mitigated, this reduction is offset by a significant rise in N2 O emissions. This is particularly concerning given that N2 O exhibits a greenhouse effect nearly 300 times greater than that of CO2 . Conclusion: It should be pointed out that numerous studies are needed to evaluate plant-wide control strategies in WWTPs for informed and effective decision-making regarding performance optimization.